1. Field of the Invention
The present invention relates to methods and apparatus for performing extracorporeal blood oxygenation and in particular to apparatus and methods which utilize membrane oxygenation.
2. The Prior State of the Art
Various heart and circulatory ailments as well as some types of serious injury can adversely affect the flow of blood through the heart and lungs thereby reducing the rate of natural blood gas exchange. Such ailments or injury cause inadequate blood gas exchange which includes both inadequate blood oxygenation and inadequate removal of carbon dioxide (CO.sub.2). Also, various types of surgical procedures necessarily require the heart and lungs to be temporarily bypassed. Accordingly, heart/lung bypass circuits have been developed which perform blood oxygenation. Such devices are capable of performing a substantially similar blood gas exchange function to that which naturally occurs in the lungs.
One type of device involves mixing small bubbles of oxygen-rich gas with blood so that the oxygen can be absorbed. Conventional bubble oxygenators developed two primary problems. First, the blood foamed during the gas exchange process. The foam had to be removed before the blood could be returned to the circulatory system. Second, the relatively extended period of time spent by the blood outside the warm confines of a patient's body required a heat exchange mechanism to maintain adequate and essential blood temperature. In the earliest configuration of known blood oxygenation devices, the heat exchanger and the oxygenator were two separate devices built into the same circuit.
Another type of extracorporeal oxygenator employs the use of a gas permeable membrane. The basic concept of operation is essentially the same in all of these membrane oxygenation devices. Blood flows in surface contact with one side of the gas permeable membrane while an oxygen-rich gas flows in surface contact with the other side of the membrane. As the blood flows through the device, the partial pressures of oxygen and carbon dioxide in the blood and in the gas cause oxygen to travel across the gas permeable membrane and to enter the blood. Simultaneously, carbon dioxide exits the blood and travels across the gas permeable membrane. Such a gas permeable membrane allows for the oxygenation of the blood without introducing oxygen bubbles into the blood thereby eliminating the need for defoaming apparatus.
There are several important design characteristics which must be taken into account in order to design an effective membrane oxygenation device. One of the more important design characteristics is the gas exchange performance of the device, characteristic reflects the ability of the device to transfer oxygen to and carbon dioxide from the blood. A well-designed device must have gas exchange characteristics which can meet the metabolic gas exchange requirements of patients up to 200 or 300 pounds in weight while undergoing cardiac surgery or otherwise requiring the use of the heart/lung bypass apparatus.
The gas exchange characteristic of a device is dependent upon several factors. One such factor is termed the "void fraction" of the device. By calculating the total volume within the membrane oxygenator and subtracting from that volume the volume that is taken up by the gas-permeable membrane, one is left with the resultant void volume. The void volume divided by the total volume constitutes the void fraction of the device. In other words, the "void fraction" can be thought of as the amount of space that is available for blood to permeate and flow through the membrane oxygenator. In the case of gas permeable membrane oxygenators which utilize woven bundles of gas permeable tubes, the void fraction is essentially a measurement of the space between the various membrane tubes in relation to the overall space which is taken up by the bundle of gas exchange tubes.
Another important factor which must be taken into account in designing the gas exchange performance characteristic of a device is the hydraulic radius. That factor reflects the dimension of the flow path through which blood has to travel. The void fraction and the hydraulic radius both effect volumetric flow rate, cross-sectional area of the flow path and the distance through which the blood must flow while it is oxygenated. This in turn determines the amount of residence time that the blood spends when travelling through the oxygenator device.
These various design considerations are typically balanced one against the other in an effort to achieve a device which is as small as possible so that the volume of blood which is needed to prime the device is decreased as much as possible, while on the other hand attempting to maintain a relatively high surface area which is available for contact by the blood so that oxygenation can occur as quickly and as efficiently as possible, while also attempting to maintain the void fraction as high as possible so as to not subject the blood to high pressure which may potentially damage the blood, or unduly long residence times.
It has been very difficult in the past to effectively balance the various design considerations and to achieve optimal performance in all of the mentioned areas, namely, low priming volume, high efficiency or gas exchange performance, and low pressure requirements.